This quote from Ozzy is the biggest factor:
"The fact that pilots are considered flawed, responsible for (typically 70%) of fatal aircraft accidents, and thus unmanned aircraft would be safer, is an untenable conclusion that ignores the vast proportion of accident or incident-free flights achieved daily by the current manned commercial air transport industry, particularly in States with well-developed aviaiton safety practice. To those who quote the 70% figure I respond: "...but 100% of non-accident flights are the result of the pilots doing their job successfully". Non-accident flights utterly dwarf the proportion of accident flights."
The simple truth is that there are no official estimates, never mind reliable statistics, to demonstrate how many occasions human pilots have prevented automated disaster. My estimate is a couple of times per flight. Not only do they dwarf the pilot error events, they would make the pilot error accidents statistically irrelevant.